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Garnet-biotite diffusion mechanisms in complex high-grade orogenic belts : understanding and constraining petrological cooling rates in granulites from Ribeira Fold Belt (SE Brazil)
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Advisor(s)
Abstract(s)
Cooling rates based on the retrograde diffusion of Fe2+ and Mg between garnet and biotite inclusions commonly show two contrasting scenarios: a) narrow
closure temperature range with apparent absence of retrograde diffusion; or b) high result dispersion due to compositional variations in garnet and biotite.
Cooling rates from migmatites, felsic and mafic granulites from Ribeira Fold Belt (SE Brazil) also show these two scenarios. Although the former can be
explained by very fast cooling, the latter is often the result of open-system behaviour caused by deformation. Retrogressive cooling during the exhumation of
granulite-facies rocks is often processed by thrusting and shearing which may cause plastic deformation, fractures and cracks in the garnet megablasts,
allowing chemical diffusion outside the garnet megablast – biotite inclusion system.
However, a careful use of garnets and biotites with large Fe/Mg variation and software that reduces result dispersion provides a good correlation between
closure temperatures and the size of biotite inclusions which are mostly due to diffusion and compositional readjustment to thermal evolution during
retrogression.
Results show that felsic and mafic granulites have low cooling rates (1–2 °C/Ma) at higher temperatures and high cooling rates (~100 °C/Ma) at lower
temperatures, suggesting a two-step cooling/exhumation process, whereas migmatites show a small decrease in cooling rates during cooling (from 2.0 to 0.5
°C/Ma). These results agree with previously obtained thermochronological data, which indicates that this method is a valid tool to obtain meaningful
petrological cooling rates in complex high-grade orogenic belts, such as the Ribeira Fold Belt.
Description
Keywords
Petrologia Termocronologia Garnet Biotite Faixa Ribeira (Brasil) Brasil (Sudeste)
Citation
Santos, Telmo M. Bento dos; Tassinari, Colombo C. G.; Fonseca, Paulo E. - Garnet-biotite diffusion mechanisms in complex high-grade orogenic belts : understanding and constraining petrological cooling rates in granulites from Ribeira Fold Belt (SE Brazil). In: Journal of South American Earth Sciences, Vol. 56 (December 2014), p. 128-138